Pink noise is noise whose level falls about 3 dB for every octave up in pitch, which puts it between white noise, which doesn't fall at all, and brown noise, which falls about 6 dB. On greensound it is made live in your browser from white noise and a filter of six parts added together.
What does 3 dB per octave mean?
An octave is a doubling of pitch: 400 Hz to 800 Hz is one octave, and 800 Hz to 1,600 Hz is the next. Pink noise gets about 3 dB quieter with every octave you go up. We measured the filter's output against that rule from 50 Hz to 10 kHz, and it stays within about 0.05 dB of a straight line. The drawing above is made from the same numbers the filter runs on.
How is pink noise made here?
It starts as the same thing white noise is on greensound: one random number per sample, generated in your browser by the audio worklet. Each number then goes through Paul Kellet's refined pink filter, which is six running sums, each leaking back toward zero at its own rate, added together with the sample itself and the one before it. Together they tilt the flat spread of white noise into one that falls about 3 dB per octave. There is no recording, sample or loop behind it, so it never repeats, and it is generated fresh every time you press play.
Pink noise or white noise?
White noise here is those random samples with nothing done to them afterward, so every hertz carries the same energy. Pink noise starts from the same samples and runs them through the filter above, so the low end keeps more of the energy and the level drops about 3 dB with each octave up. Side by side, pink has proportionally less at the top of the range than white does.
Pink noise or brown noise?
Brown noise here is built differently: a single running sum that leaks back toward zero, with its energy falling at roughly 6 dB per octave above about 150 Hz. Pink falls about 3 dB per octave, half as steeply. So pink sits between white and brown in how fast it falls, and brown, falling twice as fast, has proportionally the least at the top of the range.
Pink noise or green noise?
Green noise is white noise run through a band-pass filter centered at 500 Hz, so it is turned down at both the low end and the high end of what you can hear. Pink noise isn't centered on anything. It has no band to sit in, only the same steady fall as pitch rises.